Over the years, the types of frames used to support vehicle chassis have gone through transitions as better means have developed for giving lighter and safer constructions. In particular, a chassis construction referred to as the unitized body or unibody construction has been rapidly replacing the more conventional frame construction. In the uniframe construction, the frame members do not extend the full length of the body. They are shorter, have thinner wall thicknesses and are usually welded to the body itself. The under body section is reinforced to provide the floor with enough structural strength to replace portions of the side rail of conventional frames.
The unibody construction has developed several variations. In one type, every member is related one to the other so that all sections carry part of the load. Thus, the rocker panels, floor pans and other sections of the lower portion of the body are welded together to form a basic structure. The front of the structure where the engine and suspension are mounted is heavily reinforced and has the appearance of a separate frame except that the rails are not bolted, but welded to the body structure.
In some newer types of automobiles, in order meet the increasing demand for a better fuel mileage, yet maintain safety concerns, the vehicle has been further redesigned. The weight of the body has been reduced by using more plastics, high strength steels and aluminum. To meet continued safety concerns, the vehicles are designed with what is called a passive safety design which is specifically designed to absorb an impact as the vehicle crushes or bends. The manufacturers specifically design areas in the vehicle which are meant to bend and crush to absorb the energy of impact.
Although frame construction has changed markedly over the years, the basic idea behind frame alignment after a collision remain the same. Essentially, if a collision occurs which causes frame misalignment and damage, an attempt will be made to pull the frame back to its original position. Due to the use of lighter gauge metals throughout the unibody construction, it has been said that greater finesse is often needed during this activity. Excessive force may cause tearing and stretching of the frame members instead of correcting and straightening the damage.
In aligning the unibody frame after a collision, it is often necessary to pull directly on the thin walled box-or channel-type side rails. The attachment of the pulling chains is of particular concern to avoid the risk of tearing the steel. In the past, the chains used to pull the box or channel-type frame members into position have been attached to the pre-existing holes or flanges found on the members. In other cases, attempts have been made to grasp the outside of the box or channel-type members. These have not proved entirely satisfactory since they create narrow stress points which increase the likelihood of failure of the frame member.